The present invention relates to power tongs typically used in the oil and gas industry to make up and break apart threaded joints on pipe, casing, and similar tubular members.
Power tongs have been used for many years and are generally employed in the oil and gas industry to grip and rotate tubular members, such as tubular. The tubular members are gripped with high compressive forces while applying a high degree of torque to break apart or tighten threaded tubular member connections. In most cases, power tong designs employ a cam mechanism for converting a portion of the torque into a gripping (compressive) force normal to the tubular member. This conversion is often accomplished using a power-driven ring gear having an interior cam surface. As the ring gear rotates, cam follower (roller) on a jaw member rides upon the cam surface, causing the follower (and thus the jaw member) to move into contact with the tubular member. U.S. Pat. No. 4,404,876 discloses such an arrangement.
Most current power tong designs include a ring gear cam with an open slot or throat ("open-throat power tongs"), through which the tubular member passes to position the jaw members around the tubular member. However, some tong designs employ a ring gear cam that has no open throat and is thus a solid circular member. This solid circular ring gear design is generally referred to as a closed-head power tong.
When the drilling environment allows the use of a derrick or similar overhead structure from which a power tong may be suspended, an openthroat power tong is often a more efficient tool. Open-throat power tongs easily disengage and/or retract from the tubular member when the operator desires to raise, lower, or otherwise manipulate the tubular member.
On the other hand, a closed-head power tong is more difficult to retract from the tubular member because it has a closed throat and must pass over the end of a tubular member. However, there are common drilling environments where there is no structure from which an open-throat power tong may be suspended and insufficient workspace to engage and retract open-throat power tongs. In such environments, a closed-head power tong may be the only practical alternative. Closed-head power tongs are highly useful during operations where snubbing units are employed. Typically, closed-head power tongs are positioned over the drill string with the individual tubular members forming the drill string extending through a center aperture in the closed-head power tong. A tubular member is moved vertically through the center aperture until the threaded joints for connecting adjacent tubular members are in position to be made up (screwed together) or broken out (unscrewed).
To increase efficiency, as many successive tubular member connections as possible are made-up or broken out without interruption, i.e. having to move the center aperture of the tong out of alignment with the drill string. However, the drill string may include a down hole tool or other device that has a diameter greater than the diameter of the center aperture of the closed-head power tong. In these situations, the prior art closed-head power tongs typically require that the drill string be broken and any tubular positioned in the center aperture at that point be removed therefrom. The closed-head power tongs are then removed from alignment with the drill string, and the oversized tool is removed from the drill string or re-positioned vertically along the drill string above or below the power tong such that it is not required to pass through the power tong. The closed-head power tong can then be re-aligned with the drill string. To re-establish the connection of the drill string through the center aperture, the unconnected tubular joint must be positioned above or below the center aperture to reconnect to a continuous length of drill string.
What is need in the art is a closed-head power tong design which allows the center aperture to be readily increased in diameter without the necessity of removing the drill string from the center aperture. The closed-head power tong design should provide for center aperture enlargement with a minimum lost time and with such simplicity that unskilled workers could perform the task.